Make sure the teacher is qualified, urges Hansa Knox, president of Yoga Alliance, a national registry of yoga teachers and schools. She recommends teachers have at least 200 hours of yoga-specific training. A background solely in aerobics or other sports disciplines isn't enough. For a database of trained and experienced instructors, check out www.yogaalliance.org.

Participate in a class before signing up for the full series or session. A good instructor won't mind if you window shop, and you'll have a chance to see what kind of learning environment he or she creates.

While you're sitting in on that class, count the number of people in the room. If there are 30 or 40 people milling around, it's impossible for the teacher to make sure everyone in the room is performing the poses correctly. And as a beginner, you need lots of extra attention. Look elsewhere if you see any more than a dozen people.

Does the instructor modify the poses to fit a person's own abilities? Yoga should be adapted to the individual, points out yoga instructor Arkady Shirin, rather than the other way around. Injuries often happen when someone tries to force a pose. "Your body will complain if you don't respect its limits," he says. "In yoga, pain is no gain."